Up-regulation of M1 muscarinic receptors expressed in CHOm1 cells by panaxynol via cAMP pathway

Loss of cholinergic neurons along with muscarinic acetylcholine receptors (mAChRs) in cerebral cortex and hippocampus is closely associated with Alzheimer's disease (AD). Recent drug development for AD treatment focuses heavily on identifying M-1 receptor agonists. However, mAChRs undergo down-regulation in response to agonist-induced sustained activation. Therefore, therapeutic effectiveness wanes during continuous use. Thus, another potentially effective approach, which overcomes this drawback is to develop compounds, which instead up-regulate M-1 receptor expression. In the present study, we took this alternative approach and contrasted in Chinese hamster ovary cells transfected with human m(1) subtype gene (CHOm(1) cells) changes of M-1 receptor expression levels caused by muscarinic agonists and upregulators of its expression. The muscarinic agonists carbachol and pilocarpine reduced M-1 receptor number in CHOm(1) cells by 29 and 46%, respectively, at 100 mu M, whereas panaxynol, a polyacetylene compound isolated from the lipophilic fraction of Panax notoginseng, concentration-dependently up-regulated the M-1 receptor number after pre-incubation with CHOm(1) cells for 48 h, reaching a plateau at 1 mu M, and was accompanied by enhanced M-1 mRNA levels. Moreover, the protein kinase A (PKA) inhibitor RP-adenosine-3',5'-cyclic monophosphoro-thioate triethylamine salt (RP-cAMPs) 5 mu M completely prevented panaxynol-induced up-regulation of M-1 receptors. Panaxynol (1 mu M) caused a significant and consistent stimulation of cAMP accumulation (27% increase above basal at 40 min). These results suggest that in CHOm(1) cells panaxynol up-regulates M-1 receptor number through cAMP pathway-mediated stimulation of gene transcription. (c) 2005 Elsevier Ireland Ltd. All rights reserved.